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Well that's what I thought as well but my mechanic says that it has a minimal effect. And it makes sense to keep the old filter in right now so that any crud freed from the new fluid junks up the old...

60% of the old fluid remains in the torque converter. So by multiple drain an refills, eventually the old fluid is replaced so that the majority of fluid is new. And it's only about $30 to do a drain...

So my plan is to first drain the old fluid and refill; next, drive for about 50 -100 miles; and then last drop the pan, change the filter, clean the pan and refill again. I plan to use castrol Dexron...